In the field of magnetic recording using a head and a medium, further improvement in performance of the magnetic recording medium and the magnetic head is demanded as the recording density of magnetic disc devices increases.
A magnetic recording medium is a discontinuous medium consisting of a set of magnetic grains. The magnetic grains each have a single-domain structure. Each recording bit on a magnetic recording medium consists of a plurality of magnetic grains. Therefore, in order to increase the recording density, the magnetic grains must be smaller and the borders between adjacent recording bits must be less uneven. However, as the magnetic grains become smaller, the magnetic grains are less thermally stable in association with their reduced volume.
A presumable measure to solve the above problem is to increase the magnetic anisotropy energy Ku of magnetic grains. However, an increase in the Ku leads to an increase in the anisotropic magnetic field (magnetic coercive force) of a magnetic recording medium. On the other hand, the upper limit of the recording magnetic field intensity of a magnetic head is largely determined by the saturation magnetic flux density of the soft magnetic material constituting the magnetic core within the head. Therefore, if the anisotropic magnetic field of a magnetic recording medium exceeds an acceptable value determined based on the upper limit of the recording magnetic field intensity, recording on the magnetic recording medium fails.
Currently, a proposed method of solving the above problem of thermal stability uses energy assisted recording in which assistive energy is provided to a medium during recording to lower the effective recording magnetic field intensity for a magnetic recording medium formed by a high Ku magnetic material. The recording system using a microwave magnetic field as the assistive energy source is called microwave assisted magnetic recording (MAMR) and is being proactively studied/developed for practical use.
In microwave assisted magnetic recording, a microwave magnetic field of a frequency corresponding to the effective magnetic field (Heff) for magnetization of the recording layer of a magnetic recording medium is applied in the medium in-plane direction, whereby magnetization precession is excited in the recording layer, and the recording capability of the magnetic head is assisted.
As an exemplary magnetic head using the microwave assisted magnetic recording method, as shown in FIG. 14, a magnetic head is proposed that includes a main magnetic pole 6′ that generates a recording magnetic field for application to a magnetic recording medium 100′, a trailing shield 7′, and a spin torque oscillator (STO) 10′ that is provided between them (write gap) and that has a multilayered a magnetic thin film structure, and a leading shield 8′. In the magnetic head, a microwave magnetic field HM in the in-plane direction is generated due to the self-excited oscillation of an STO 10′, precession movement of the magnetization of the recording layer is induced by applying the microwave magnetic field HM to the magnetic recording medium 100′, and magnetization reversal in the perpendicular direction in the recording layer is assisted.
In such a magnetic head, in general, a direct current (drive current) is always applied to the spin torque oscillator 10′ while the writing operation is executed. The recording current waveform for generating a recording magnetic field HW from the main magnetic pole 6′ in recording signals on the magnetic recording medium 100′ using such a magnetic head usually includes relatively long polarity reversal intervals and relatively short polarity reversal intervals. Here, a polarity reversal interval means the interval from one polarity reversal time to the next polarity reversal time. There is a problem that the writing operation according to a recording current of a relatively long polarity reversal interval deteriorates the bit error rate (BER) of signals recorded on the magnetic recording medium 100. The BER deterioration is presumably caused by the returning magnetic field HR (the magnetic field in the opposite direction to the recording magnetic field), which is the recording magnetic field HW from the main magnetic pole 6′ that returns and occurs near the end face on the ABS side of the trailing shield 7′ as shown in FIG. 14. In other words, as the returning magnetic field HR and the microwave magnetic field HM generated from the spin torque oscillator 10′ are applied to the magnetic recording medium 100′ in a superimposed manner, problems such as deletion or deterioration of signals recorded on the magnetic recording medium 100′ and reduced quality of recorded signals occur.
In order to solve the above problem, a microwave assisted magnetic head is proposed that controls the means for supplying a direct current to the spin torque oscillator to turn off the direct current to the spin torque oscillator when the number of times of polarity reversal (number of times of magnetization reversal) in a recording current waveform within a given time interval is equal to or lower than reference value, and to turn on the direct current to the spin torque oscillator when the number of times of magnetization reversal exceeds reference value (JP Laid-Open Patent Application No. 2014-211933).
In the above microwave assisted magnetic head, it is determined whether to turn on or off the direct current to the spin torque oscillator taking into consideration as an indicator the number of times of polarity reversal (number of times of magnetization reversal) in a recording current waveform within a given time interval. Therefore, depending on the setting of the given time interval for the recording current waveform, the direct current may be applied to the spin torque oscillator even in recording a signal corresponding to a relatively long polarity reversal interval, whereby it is difficult to solve the problem of lowered quality of signals recorded on a magnetic recording medium.